This invention relates to multichannel subscription television systems utilizing scrambled television signals for providing secure transmission of selected-priority programs to subscriber equipment, and more particularly to such systems in which timing reference signals are transmitted to subscriber receiver equipment for use in descrambling associated priority television channel signals having suppressed horizontal synchronization and blanking puses.
Subscription-type CATV systems are described in U.S. Pat. Nos. 2,907,816, Subscription Television System by P. H. Weiss, dated Oct. 6, 1959 and 3,201,511, Subscription Television System Having Keyed Generation of Sync Signals at the Receiver, by H. Doundoulakis, dated Aug. 17, 1965. Since all customers of a cable television company will not elect to subscribe to priority programs, it is necessary in pay-cable television systems to scramble priority channel television signals so that they will not produce a viewable picture on a television screen. Only after payment of a service charge and installation of descrambler equipment at a subscriber's facility is a viewable television picture obtained on a television receiver there for the restricted or pay television channels.
One method of scrambling a composite television signal in head-end transmission equipment is to attenuate horizontal synchronization and blanking pulses thereof prior to launching it on a distribution system that may comprise a coaxial cable network. An associated timing signal having a frequency and phase related to that of the suppressed horizontal sync (i.e., synchronization and blanking) pulses is also transmitted to subscriber equipment where it is used in a descrambler for restoring horizontal sync pulses to their proper amplitude and sequence positions in the composite television signal. In one known technique, timing pulses are amplitude modulated onto a high frequency carrier signal for transmission on a coaxial cable. This technique requires a relatively wide frequency bandwidth for transmission that faithfully reproduces the rise and fall times of the square wave timing pulses in the receiver. In a system that uses microwave relays, for example, the wide bandwidth requirement necessitates dedication of a separate television type RF transmission channel for each timing signal. This is costly, especially in systems where signals are scrambled for a plurality of television channels that require separate timing information channels. Also, wide-band signals are difficult to detect in the presence of noise. In another technique employing a sinusoidal timing signal, a continuous wave audio signal is transmitted over a dedicated channel such as a telephone line. In yet another technique, a sinusoidal timing signal is amplitude modulated on a carrier signal for transmission. The amplitude modulated carrier signal is combined with the local oscillator signal that is generated in a subscriber's television receiver for reproducing a sinusoidal timing signal that is used in company-owned equipment there for descrambling the television signal. It is not desirable for a CATV operating company to utilize or modify a subscriber's television receiver, other than to connect an external cable to it, since this may subject the company to liability if a subscriber is injured when touching his television set or when the latter becomes defective. Also, to require a separate local oscillator, such as a temperature stable crystal oscillator, in each subscriber's location is costly.
An object of this invention is the provision of improved timing circuitry in a subscription television system.